Magnetographic printing is a relatively recent but well known technique. The magnetographic printing process operates by transferring an image which is be printed onto a magnetic drum with the image being represented on the cylindrical surface of the drum by magnetized portions or areas (really small spots) on the drum surface. The magnetized portions are provided by utilizing one or more magnetic recording heads. The latent image on the drum is then toned with magnetic particles. Thereafter, the toned imaged image is transferred to a sheet of paper or other material and is then fixed to the paper. A great advantage of magnetographic printing is that it is a high speed operation capable of outputs of 600 linear feet per minute. One commercial apparatus for carrying out magnetographic printing is the Cynthia MP 6090 Page Printer (trademark) sold by Bull Peripheriques and described in detail in Cynthia MP 6090 Page Printer Product Specification, 1985, published by Bull Peripheriques Direction.
In the case of development of electrostatic images an electric field may be used to cause ink, for example, on an analog roller to extend and deposit on a drum. Such is discussed, for example, in U.S. Pat. Nos. 4,202,620; 4,202,913 and 4,268,597 of I.L. Klavan, et al. In a magnetographic printing process wherein particles are attracted to selectively magnetized (not electrically charged) portions of a drum, such a technique is not applicable and has not been used. Indeed, limitation on magnetographic printing has been the fact that the toner is in the nature of powdered magnetic iron oxide. This material is quite satisfactory for printing in black, but is highly unsatisfactory for printing in any other color. Thus, high speed quality color printing has not been possible using this technique. Attempts have been made to coat the iron oxide with a white coating and then with a coating of desired color to allow the production of multicolored images, or at least images in other colors besides black. Such attempts have had, at best, limited success since the magnetic iron oxide particles are quite black whereby providing a white coating followed by a coating of a desired color will generally not completely block out the blackness of the iron oxide. As a result the color has a significant grayish or blackish tinge and is far from bright. If sufficient white coating and then coloring is placed over an iron oxide particle so that the desired color is sufficiently bright, the coatings must be so thick that the particles cannot be reasonably used for magnetographic printing.
Color printing by typical electrostatic processes produces acceptable colored images only at a low rate of speed, perhaps about 6 linear feet per minute. It would be very desirable if it were possible to produce acceptable colored images at high magnetographic printing output rates.
The present invention is directed to overcoming one or more of the problems as set forth above.